In the construction of pneumatic tires, it is believed to be virtually impossible to economically manufacture an absolutely uniform tire because of the many variables involved in a tire's construction. Consequently, pneumatic tires, as manufactured, almost inevitably possess a certain degree of non-uniformity. The effects of non-uniformity are best explained by noting that several types of forces are simultaneously exerted by a tire during its rotation under load against a surface. For example, radial forces, which are of particular importance in the present application, are exerted in the radial direction of the tire, or in a direction perpendicular to its plane of rotation. Additionally, lateral forces, which are also of particular importance in the present application, are exerted in the axial direction of the tire or in a direction parallel to its plane of rotation. In a non-uniform tire, the radial and lateral forces exerted by the tire will vary or change during its rotation. In other words, the magnitude and/or direction of the radial and lateral forces exerted by the tire will depend on which increment of its tread is contacting the surface.
The variations in radial and lateral force during rotation of a tire is usually caused by differences in the stiffness and/or geometry of the tire about its circumference, or tread. If these differences are slight, the radial and lateral force variations will be insignificant and their effects unnoticeable when the tire is installed on a vehicle. However, when such differences reach a certain level, the radial and/or lateral force variations may be significant enough to cause rough riding conditions and/or difficult handling situations.
Consequently, methods have been developed in the past to correct for excessive force variations by removing material from the shoulders of the tire. Most of these correction methods include the steps of indexing the tire tread into a series of circumferential increments and obtaining a series of force measurements representative of the force exerted by the tire as these increments contact a surface. This data is then interpreted and material is removed from the tire tread in a pattern related to this interpretation. These methods are commonly performed with a tire-uniformity machine which includes an assembly for rotating a test tire against the surface of a freely rotating loading drum. This arrangement results in the loading drum being moved in a manner dependent on the forces exerted by the rotating tire whereby forces may be measured by appropriately placed measuring devices. In a sophisticated tire-uniformity machine, the force measurements are interpreted by a computer and material is removed from the tire tread by grinders controlled by the computer.
Some correction methods are designed to correct only for excessive radial force variations and any excessive lateral force variations are ignored. (See e.g., U.S. Pat. No. 4,914,869; U.S. Pat. No. 3,914,907; U.S. Pat. No. 3,849,942; U.S. Pat. No. 3,848,368; U.S. Pat. No. 3,817,003; U.S. Pat. No. 3,724,137; U.S. Pat, No. 3,681,877; U.S. Pat. No. 3,574,973; U.S. Pat. No. 3,553,903; and U.S. Pat. No. 3,491,493.) Other correction methods are designed to correct only for excessive lateral force variations and any excessive radial force variations are ignored. (See e.g., U.S. Pat. No. 4,047,338; U.S. Pat. No. 3,946,527; and U.S. patent application No. 07/833,378.) Consequently, these methods do not simultaneously correct for both excessive radial force variations and excessive lateral force variations.
The present invention provides a method of simultaneously, efficiently, and accurately, correcting excessive radial force variations and excessive lateral force variations in a pneumatic tire. The method is preferably performed with a tire uniformity machine including a freely rotating loading drum, an assembly which rotates the tire against the loading drum, measurement devices which measure the lateral force and radial force exerted by the tire as it rotates against the loading drum, grinders which are adapted to move into and out of cutting engagement with the tire tread shoulders, and a computer which interprets the measurements and which controls the grinders.
More particularly, the present invention provides a correction method including the steps of indexing the tire tread into a series of circumferential increments I(n), and obtaining a corresponding series of radial force measurements R(n) and a corresponding series of lateral force measurements L(n). A radial grind component R.sub.$GR (n) is then generated, for each of the increments I(n), which represents the amount of material removal necessary to correct excessive radial force variations. A first lateral grind component L.sup.1.sub.$GR (n) and a second lateral grind component L.sup.2.sub.$GR (n) are also generated for each of the increments I(n). The first lateral grind component L.sup.1.sub.$GR (n) represents the amount of material removal on the first shoulder necessary to correct excessive lateral force variations, and the second lateral grind component L.sup.2.sub.$GR (n) represents the amount of material removal on the second shoulder of the tire tread necessary to correct excessive lateral force variations.
The radial grind component R.sub.$GR (n) and the lateral grind components L.sup.1.sub.$GR (n) and L.sup.2.sub.$GR (n) are used to generate a first radial/lateral grind component R/L.sup.2.sub.$GR (n) and a second radial/lateral grind component R/L.sup.2.sub.$GR (n) for each of the increments I(n). The first radial/lateral grind component R/L.sup.1.sub.$GR (n) represents the amount of material removal from the first shoulder region of the increment I(n) which is necessary to simultaneously correct excessive radial and lateral force variations in the tire. The second radial/lateral grind component R/L.sup.2.sub.$GR (n) represents the amount of material removal from the second shoulder region of the increment I(n) which is necessary to simultaneously correct excessive radial and lateral force variations in the tire. Material is then removed from the first and second shoulder regions of each increment I(n) according to the value of the corresponding first radial/lateral grind component R/L.sup.1.sub.$GR (n) and the value of the corresponding second radial/lateral grind component R/L.sup.2.sub.$GR (n), respectively. In this manner, an increment-by-increment analysis of both excessive radial force variations and excessive lateral force variations is used in the generation of the radial/lateral grind components R/L.sup.1.sub.$GR (n) and R/L.sup.2.sub.$GR (n). Such an analysis is believed to accurately, efficiently, and simultaneously correct for excessive radial force variations and excessive lateral force variations in a pneumatic tire.
Preferably, the step of generating the first radial/lateral grind component R/L.sup.1.sub.SGR (n) includes adding the corresponding radial grind component R.sub.$GR (n) and the corresponding first lateral grind component L.sup.1.sub.$GR (n). The step of generating the second radial/lateral grind component R/L.sup.2.sub.$GR (n) preferably includes adding the negative of the corresponding radial grind component R.sub.$GR (n) and the corresponding second lateral grind component L.sup.1.sub.$GR (n).
More preferably, the step of generating the radial/lateral grind components R/L.sup.1.sub.$GR (n) and R/L.sup.2.sub.$GR (n) further includes generating, for each increment I(n), a radial/lateral multiplier R/L.sub.multiplier, (n). The sum of the radial grind component R.sub.$GR (n) and the first lateral grind component L.sup.1.sub.$GR (n) is multiplied by the corresponding radial/lateral multiplier R/L.sub.multiplier (n) to generate the first radial/lateral grind component R/L.sup.1.sub.$GR (n). The sum of the negative of the radial grind component R.sub.$GR (n) and the second lateral grind component L.sup.2.sub.$GR (n) is multiplied by the corresponding radial/lateral multiplier R/L.sub.multiplier (n) to generate the first radial/lateral grind component R/L.sup.1.sub.$GR (n).
Even more preferably, the grind factors R.sub.$GN (n), L.sup.1.sub.$GR (n), and L.sup.2.sub.$GR (n) are used to generate the radial/lateral multiplier R/L.sub.multiplier (n). Specifically, if, for a particular increment I(n), the radial grind component R.sub.$GR (n) is equal to zero or if either of the lateral grind components L.sup.1.sub.$GR (n) or L.sup.2.sub.$GR (n) is equal to zero; the corresponding radial/lateral multiplier R/L.sub.multiplier (n) is set equal to a default value of 1.0. (In such a situation, the profile of this particular increment I(n) is responsible for either (but not both) excessive radial force variations or excessive lateral force variations in the tire.) However, if, for a particular increment I(n), the radial grind component R.sub.$GR (n) is a non-zero value and either of the lateral grind components L.sup.1.sub.$GR (n) or L.sup.2.sub.$GR (n) is a non-zero value, the corresponding R/L.sub.multiplier is set equal to a fractional value R/Lf. (In such a situation, the profile of this particular increment I(n) is responsible for both excessive radial force variations and excessive lateral force variations in the tire.)
In the past, methods have been developed in an attempt to correct both excessive radial force variations and excessive lateral force variations in a tire. For example, in U.S. Pat. No. 3,948,004 to Gruber, a method is disclosed in which a resultant vector is created representing radial and lateral force variations. The tangent angle .alpha. of this vector (which is equal to the tangential inverse of the lateral force variation divided by the radial force variation) is used to determine the appropriate material removal pattern. Specifically, if the angle .alpha. is less than 45.degree. (and thus the radial force variation exceeds the lateral force variation) material is mainly removed in the radial direction by approaching two grinders to the tire shoulders with the same force. If the angle .alpha. is between 45.degree. and 90.degree. (and thus the radial force variation is less than or equal to the lateral force variation) material is mainly removed in the lateral direction with one of the grinders approaching the tire with a force exceeding that of the other grinder on every other rotation of the tire. Thus, the Gruber method appears to correct for either excessive radial force variations or lateral force variations, depending on which variations are dominant (i.e. depending on whether the angle .alpha. is less than or greater than 45.degree.).
Additionally, U.S. Pat. No. 3,739,533 to Iida et al. discloses a correction method comprising the steps of generating first and second composite signals, and removing the shoulder rubber of the tire in response to these composite signals. In generating the composite signals, the Iida method determines: a radial force variation corrective signal which represents the difference between the measured radial force variation and a predetermined maximum allowable limit; a "positive" lateral force deviation corrective signal which is proportional to the magnitude of the mean of positive lateral force variations; and a "negative" lateral force deviation corrective signal which is proportional to the magnitude of the mean of negative lateral force variations. The first composite signal is generated by adding the radial force variation corrective signal and the "positive" lateral force deviation corrective signal. The second composite signal represents the sum of the radial force variation correction signal and the absolute value of the "negative" lateral force deviation corrective signal. Thus, the Iida method only take into account the mean of the lateral force deviations when generating its composite signals.
Applicant believes that the correction method of the present invention simultaneously corrects for excessive radial force variations and excessive lateral force variations in a pneumatic tire in manner which is more efficient and/or more accurate than the simultaneous correction methods of the prior art. Specifically, in contrast to the Gruber method, both excessive radial force variations and excessive lateral force variations are used in the generation of the radial/lateral grind components R/L.sup.1.sub.$GR (n) and R/L.sup.2.sub.$GR (n). Furthermore, in contrast to the Iida method, an increment-by-increment analysis of excessive lateral force variations (rather than simply a mean of lateral force deviations) is used in the generation of the radial/lateral grind components R/L.sup.1.sub.$GR (n) and R/L.sup.2.sub.$GR (n).
These and other features of the invention are fully described and particularly pointed out in the claims. The following description and annexed drawings set forth in detail one illustrative embodiment of the invention. However this embodiment is indicative of but one of the various ways in which the principles of the invention may be employed.